The present invention relates to semiconductor devices. It finds particular application in the context of PIN diodes but can also be used in conjunction with other diodes and indeed semiconductor devices other than diodes. The present invention will mainly be described with reference to a PIN diode, and it will be clear to one skilled in the art how the technique would need to be adapted when used with devices other that PIN diodes.
PIN diodes can be used for capturing light signals in the spectrum from red (˜650 nm) to infra-red (˜1.2 micrometer), but not exclusive to that range of wavelengths. PIN diodes comprise a P and N region of semiconductor at the ends of an intrinsic region (I). Efficient operation of a PIN diode requires an extended width of intrinsic silicon which enables capture of a substantial part of the optical energy which typically lies in the region of 10 to 50 microns for around 800 nm wavelengths. In order to optimize operation of these diodes a depletion region is required to extend across the whole length of the region from the P to the N junction. Depending on the thickness and doping level of the intrinsic region, which is rarely intrinsic but often very lightly doped (purely intrinsic would require silicon built to an impurity level of less than 1 part in a trillion (1 in 1012) and difficult to achieve in practice) an external voltage is applied to the diode so as to increase the width of the depletion region until the full width of the intrinsic or lightly doped region is encompassed. If a silicon layer with a resistivity of a few hundred ohms·cm or more is deposited over a heavily doped substrate, voltages to fully deplete the layer can be in the region of 1 to 10 V, although other voltages can be used.